> The list of disabled config option is now:>> - CONFIG_HIGHMEM [ see the mess it created in 33-rt ]

Could someone please point me to information on this?

In our setup, we do use PREEMPT_RT + HIGHMEM, on .33 for now (but want to upgrade because of new hardware support issues with .33). Up to now, we did not face any issues related to PREEMPT_RT + HIGHMEM combination.